Nozzle misalignment compensation in a drop on demand printhead

ABSTRACT

A method and apparatus is provided for compensating for variable overlap between segments of a page width print head system to reduce visually perceptible artifacts due to misalignment of adjacent overlapped segments. One method employs a summation means which sums a current dither value from a dither matrix with an overlap signal to provide an output value which is then compared in a comparator with an input continuous tone data value providing an output compensated dither value to control nozzles in the overlap region of the segments. Another method uses a software program to provide the compensated dither matrix. A sensing means provides a measure of the degree of overlap of the segments to generate the overlap signal. The sensing means may sense temperature or relative displacement of the segments. The degree of overlap may be determined for various temperatures and stored in a ROM.

[0001] Continuation Application of U.S. Ser. No. 09/575,117 on May 23,2000

FIELD OF THE INVENTION

[0002] The present invention relates to the field of ink jet printingand in particular discloses a method and apparatus for the compensationfor the time varying nozzle misalignment of a print head assembly havingoverlapping segments.

CO-PENDING APPLICATIONS

[0003] Various methods, systems and apparatus relating to the presentinvention are disclosed in the following co-pending applications filedby the applicant or assignee of the present invention simultaneouslywith the present application: 09/575,197 09/575,195 09/575,15909/575,132 09/575,123 09/575,148 09/575,130 09/575,165 09/575,15309/575,118 09/575,131 09/575,116 09/575,144 09/575,139 09/575,18609/575,185 09/575,191 09/575,145 09/575,192 09/575,181 09/575,1939/575,156 09/575,183 09/575,160 09/575,150 09/575,169 09/575,18409/575,128 09/575,180 09/575,149 09/575,179 09/575,133 09/575,14309/575,187 09/575,155 09/575,196 09/575,198 09/575,178 09/575,16409/575,146 09/575,174 09/575,163 09/575,168 09/575,154 09/575,12909/575,124 09/575,188 09/575,189 09/575,162 09/575,172 09/575,17009/575,171 09/575,161 09/575,141 09/575,125 09/575,142 09/575,14009/575,190 09/575,138 09/575,126 09/575,127 09/575,158 09/575,11709/575,147 09/575,152 09/575,176 09/575,151 09/575,177 09/575,17509/575,115 09/575,114 09/575,113 09/575,112 09/575,111 09/575,10809/575,109 09/575,182 09/575,173 09/575,194 09/575,136 09/575,11909/575,135 09/575,157 09/575,166 09/575,134 09/575,121 09/575,13709/575,167 09/575,120 09/575,122

[0004] The disclosures of these co-pending applications are incorporatedherein by cross-reference.

BACKGROUND OF THE INVENTION

[0005] In the applicant's co-pending application PCT/AU98/00550, aseries of ink jet printing arrangements were proposed for printing athigh speeds across a page width employing novel ink ejection mechanisms.The disclosed arrangements utilized a thermal bend actuator built aspart of a monolithic structure.

[0006] In such arrangements, it is desirable to form larger arrays ofink ejection nozzles so as to provide for a page width drop on demandprint head. Desirably, a very high resolution of droplet size isrequired. For example, common competitive printing systems such asoffset printing allow for resolutions of one thousand six hundred dotsper inch (1600 dpi). Hence, by way of example, for an A4 page print headwhich is eight inches wide, to print at that resolution would requirethe equivalent of around 12800 ink ejection nozzles for each colour.Assuming a standard four colour process, this equates to approximatelyfifty one thousand ink ejection nozzles. For a six colour processincluding the standard four colours plus a fixative and an IR ink thisresults in 76800 ink ejection nozzles. Unfortunately, it is impracticalto make large monolithic print heads from a contiguous segment ofsubstrate such as a silicon wafer substrate. This is primarily a resultof the substantial reduction in yield with increasing size ofconstruction. The problem of yield is a well studied problem in thesemi-conductor industry and the manufacture of ink jet devices oftenutilizes semi-conductor or analogous semi-conductor processingtechniques. In particular, the field is generally known as Micro ElectroMechanical Systems (MEMS). A survey on the MEMS field is made in theDecember 1998 IEEE Spectrum article by S Tom Picraux and Paul JMcWhorter entitled “The Broad Sweep of Integrated Micro Systems”.

[0007] One solution to the problem of maintaining high yields is tomanufacture a lengthy print head in a number of segments and to abut oroverlap the segments together. Unfortunately, the extremely high pitchof ink ejection nozzles required for a print head device means that thespacing between adjacent print head segments must be extremelyaccurately controlled even in the presence of thermal cycling undernormal operational conditions. For example, to provide a resolution ofone thousand six hundred dots per inch a nozzle to nozzle separation ofabout sixteen microns is required.

[0008] Ambient conditions and the operational environment of a printhead may result in thermal cycling of the print head in the overlapregion resulting in expansion and contraction of the overlap betweenadjacent print head segments which may in turn lead to the production ofartifacts in the resultant output image. For example, the temperature ofthe print head may rise 25° C. above ambient when in operation. Theassembly of the print head may also be made of materials havingdifferent thermal characteristics to the print head segments resultingin a differential thermal expansion between these components. Thesilicon substrate may be packaged in elastomer for which the respectivethermal expansion coefficients are 2.6×10⁻⁶ and 20×10⁻⁶ microns perdegree Celsius.

[0009] Artifacts are produced due to the limited resolution of the printhead to represent a continuous tone image in a binary form and theability of the human eye to detect 0.5% differences in colour ofadjacent dots in an image.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide for amechanism for compensating for relative displacement of overlappingprint head segments during operation in an effective and convenientmanner.

[0011] In accordance with a first aspect of the invention there isprovided in an ink ejection print head comprising a plurality ofoverlapping print head segments, wherein the spatial relationshipbetween adjacent segments is variable with time, a method forcontrolling the firing of nozzles within the overlapped segmentscomprising the steps of: (a) determining a measure of the overlapbetween adjacent print head segments; (b) creating a half toning patternfor the nozzles in the region of overlap of the overlapping segments;and (c) adjusting said half toning pattern as a function of said measurein the overlapping regions of said print head segments to reduceartifacts produced by the overlapping of said print head segments.

[0012] Preferably, the step for determining a measure of overlap employsa measure of temperature of the print head segments The half toningpatterns are preferably produced by means of a dither matrix or dithervolume and the alteration can comprise adding an overlap value to acurrent continuous tone pixel output value before utilizing the dithermatrix or dither volume. In place of a measure of temperature a measureof distance can be provided by the use of fiduciary strips on each ofthe segments and using an interferometric technique to determine thedegree of relative movement between the segments.

[0013] In accordance with a further aspect of the present invention,there is provided an ink ejection print head system comprising: aplurality of spaced apart spatially overlapping print head segments; atleast one means for measurement of the degree of overlap betweenadjacent print head segments; means for providing a half toning of acontinuous tone image and means for adjusting said half toning means ina region of overlap between adjacent print head segments to reduceartifacts between said adjacent segments.

[0014] The means for adjusting the half toning means can include acontinuous tone input, a spatial overlap input and a binary input, thehalf toning means utilizing the spatial overlap input to vary thecontinuous tone input to produce a varied continuous tone input forutilization in a look-up table of a dither matrix or dither volume so asto produce output binary values to adjust for the regions of overlap ofprint head segments. The means for adjusting the half tone or dithermatrix may be implemented in hardware or by means of software employingan algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] This invention is pointed out with particularity in the appendedclaims. The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

[0016]FIG. 1 shows a schematic of a pair of adjacent print head segmentsaccording to the invention;

[0017]FIG. 2 illustrates the process for printing dots from adjacentprint head segments as shown in FIG. 1;

[0018]FIG. 3 illustrates a process of blending dots between adjacentprint head segments according to the invention;

[0019]FIG. 4 illustrates a process of dither matrix variational controlaccording to an embodiment of the invention;

[0020]FIG. 5 illustrates a process of dither matrix variational controlaccording to another embodiment of the invention; and

[0021]FIG. 6 illustrates graphically an algorithm implementing a furtherprocess of dither matrix variational control according to a furtherembodiment of the invention.

[0022]FIG. 7 shows a schematic of a pair of adjacent printhead segmentsaccording to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] In a first embodiment, a method of compensation for thetemperature varying relative displacement of adjacent print headsegments is provided by the utilization of a digital processingmechanism which adjusts for the overlap between adjacent segments.

[0024] In a print head covering an A4 page width there may be 10segments having 9 overlapping portions arranged in a repeating sequenceof staggered pairs. Initial alignment of segments can be made within 10microns using techniques well known in the art of monolithic fabricationtechniques. The width of a segment for a 6 colour ink arrangement wouldbe approximately 225 microns assuming the nozzles of a segment arearranged on 16 micron centres in a zig-zag pattern longitudinally.

[0025] In this embodiment, a temperature sensor is placed on each printhead segment so as to provide for a measure of the current temperaturecharacteristics of each print head segment. The current temperaturemeasurement can then be utilized to determine the amount of overlapbetween adjacent print head segments.

[0026] Alternatively, only a single temperature sensor can be used if itcan be assumed that the segments of the print head are sufficientlysimilar to one another in physical characteristics and performance andthat the ambient milieu of each pair of overlapped segment issubstantially the same.

[0027] The degree of overlap is then used to provide a mechanism forcontrolling the half toning between adjacent print head segments. It isassumed that outputting of an image in the instant invention is by meansof digital half toning employing any method or technique well known inthe art. Many different half toning techniques can be utilized andreference is made to the text by Ulichney entitled “Digital Half Toning”published by MIT Press.

[0028] As shown in FIG. 1 adjacent print head segments 2, 3 overlap inthe respective regions 12, 13. The overlap region may extendapproximately 40 thou (˜1 mm.) providing an overlap of 64 nozzles spacedat 16 microns for 1600 dpi resolution.

[0029] A temperature sensor 16 is placed on each print head segment 2, 3so as to provide for a measure of the current temperaturecharacteristics of each print head segment 2, 3. The current temperaturemeasurement can then be utilized to determine the amount of overlapbetween adjacent print head segments. Alternatively, fiduciary strips100, 101 on each overlapped segment 102, 103, as shown in FIG. 7, may beused to measure the degree of relative displacement of the segments 102,103 by an interferometric technique.

[0030] In the region 10 of the segment 2 the nozzles of this segment areused exclusively for the ejection of ink. Similarly in the region 11 ofthe segment 3 the nozzles of this segment are used exclusively for theejection of ink. In the overlapping regions 12, 13 a “blend” is providedbetween the two print head segments 2, 3 such that along the edge 14 ofthe print head segment 2 nozzles are used exclusively in the region 12to print and similarly along the edge 15, the nozzles of the segment 3are used almost exclusively for printing. In between, an interpolation,which can be linear or otherwise, is provided between these two extremepositions. Hence, as shown in FIG. 2, when printing a full colour outputon a page the area on the side 17 is printed exclusively by the printhead segment 10 while the area 18 is printed exclusively by the printhead segment 11 (as illustrated by the black dots) with the area 19comprising a blend between the nozzles of the two segments. The printingprocess utilizes any well known half toning matrix such as disclosed inthe aforementioned references. While a known half toning matrix isutilized, the actual print head segment utilized will depend upon theblending ratio provided by the measure of overlap between theoverlapping segments.

[0031] One such method is illustrated in FIG. 3 where a linearinterpolation within the overlapped regions is shown. In the regioncorresponding to the overlapped section 12 at the edge 14 there is 100%utilization of the nozzles of print head segment 2, whereas in theequivalent region, edge 7, of the print head segment 3 there is zerooutput. As the distance of the overlap region from the line 14 of thesegment 2 is increased towards the line 15 of the segment 3 theproportion of utilization of the nozzles of the section 12 is graduallydecreased (linearly), being zero at edge 9 while the utilization of thenozzles of the section 13 is progressively increased to unity by thetime the edge 15 is reached. In a first embodiment, where there is anincreased overlap between nozzles, the half toning thresholds utilizedare increased in the overlap region. This reduces the number of dotsprinted in the blend region. Conversely, if there is a reduced overlapwith the print head segments being spaced apart slightly more thannormally acceptable, the dot frequency can be increased by reducing thehalf toning threshold.

[0032] An overall general half toning arrangement can be provided asshown in FIG. 4 with a dither matrix 25 outputting a current dithervalue 26 to a summation means 27 with summation means 27 having anotherinput 28, an overlap signal, which varies in either a positive or anegative sense depending on the degree of overlap between the adjacentsegments. The output value 29 of summation means or adder 27 is comparedto the input continuous tone data 32 via a comparator 30 so as to outputhalf tone data 31. An alternative arrangement allows that the data value28 can be subtracted from the continuous tone data 29 before ditheringis applied producing similar results. This arrangement is shown in FIG.5.

[0033] As shown in FIG. 5, a halftone data output 52 can be generated bycombining the output 42 of dither matrix 40 in an adder 46 with theoverlap signal 44, and then taking the difference of the output 54 ofadder 46 and the continuous tone data 48 in subtracter 50. This is anequivalent arrangement to that of FIG. 4.

[0034] Through the utilization of an arrangement such as described abovewith respect to FIGS. 3 and 4, a degree of control of the overlapblending can be provided so as to reduce the production of streakartifacts between adjacent print head segments.

[0035] As each overlap signal 28 can be multiplied by a calibrationfactor and added to a calibration offset factor, the degree of accuracyof placement of adjacent print head segments can also be dramaticallyreduced. Hence, adjacent print head segments can be roughly alignedduring manufacture with one another. Test patterns can then be printedout at known temperatures to determine the degree of overlap betweennozzles of adjacent segments. Once a degree of overlap has beendetermined for a particular temperature range a series of correspondingvalues can be written to a programmable ROM storage device so as toprovide full offset values on demand which are individually factored tothe print head segment overlap.

[0036] A further embodiment of the invention involves the use of asoftware solution for reducing the production of artifacts betweenoverlapped segments of the print heads. A full software implementationof a dither matrix including the implementation of an algorithm foradjusting variable overlap between print head segments is attached asappendix A. The program is written in the programming language C. Thealgorithm may be written in some other code mutatis mutandis within theknowledge of a person skilled in the art. The basis of the algorithm isexplained as follows.

[0037] A dispersed dot stochastic dithering is used to reproduce thecontinuous tone pixel values using bi-level dots. Dispersed dotdithering reproduces high spatial frequency, that is, image detail,almost to the limits of the dot resolution, while simultaneouslyreproducing lower spatial frequencies to their full intensity depth whenspatially integrated by the eye. A stochastic dither matrix is designedto be free of objectionable low frequency patterns when tiled across thepage.

[0038] Dot overlap can be modelled using dot gain techniques. Dot gainrefers to any increase from the ideal intensity of a pattern of dots tothe actual intensity produced when the pattern is printed. In ink jetprinting, dot gain is caused mainly by ink bleed. Bleed is itself afunction of the characteristics of the ink and the printing medium.Pigmented inks can bleed on the surface but do not diffuse far insidethe medium. Dye based inks can diffuse along cellulose fibres inside themedium. Surface coatings can be used to reduce bleed.

[0039] Because the effect of dot overlap is sensitive to thedistribution of the dots in the same way that dot gain is, it is usefulto model the ideal dot as perfectly tiling the page with no overlap.While an actual ink jet dot is approximately round and overlaps itsneighbours, the ideal dot can be modelled by a square. The ideal andactual dot shapes thus become dot gain parameters.

[0040] Dot gain is an edge effect, that is it is an effect whichmanifests itself along edges between printed dots and adjacent unprintedareas. Dot gain is proportional to the ratio between the edge links of adot pattern and the area of the dot pattern. Two techniques for dealingwith dot gain are dispersed dot dithering and clustered dot dithering.In dispersed dot dithering the dot is distributed uniformly over anarea, for example for a dot of 50% intensity a chequer board pattern isused. In clustered dot dithering the dot is represented with a singlecentral “coloured” area and an “uncoloured” border with the ratio of thearea of “coloured” to “uncoloured” equalling the intensity of the dot tobe printed. Dispersed dot dithering is therefore more sensitive to dotgain than clustered dot dithering.

[0041] Two adjacent print head segments have a number of overlappingnozzles. In general, there will not be perfect registration betweencorresponding nozzles in adjacent segments. At a local level there canbe a misregistration of plus or minus half the nozzle spacing, that isplus or minus about 8 microns at 1600 dpi. At a higher level, the numberof overlapping nozzles can actually vary.

[0042] The first approach to smoothly blending the output across theoverlap bridge and from one segment to the next consists of blending thecontinuous tone input to the two segments from one to the other acrossthe overlap region. As output proceeds across the overlap region, thesecond segment receives an increasing proportion of the input continuoustone value and the first segment receives a correspondingly decreasingproportion as described above with respect to FIG. 3. A linear or higherorder interpolation can be used. The dither matrices used to dither theoutput through the two segments are then registered at the nozzle level.

[0043] The first approach has two drawbacks. Firstly, if the ditherthreshold at a particular dot location is lower than both segments'interpolated continuous tone values then both segments will produce adot for that location. Since the two dots will overlap, the intensitiespromised by the two dither matrices will be only partially reproduced,leading to a loss of overall intensity. This can be remedied by ensuringthat corresponding nozzles never both produce a dot. This can also beachieved by using the inverse of the dither matrix for alternatingsegments, or dithering the continuous tone value through a single dithermatrix and then assigning the output dot to one or the other nozzlestochastically, according to a probability given by the currentinterpolation factor.

[0044] Secondly, adjacent dots printed by different segments willoverlap again leading to a loss of overall intensity.

[0045] As shown in FIG. 6, the value for each overlapped segment isplotted along the horizontal axes 60, 62 as V_(A) and V_(B) respectivelybetween the values of 0.0 and 1.0. The calculated output 66 is plottedwith respect to the vertical axis 64 as a function, I_(A+B), for valuesranging from 0.0 to 1.0. A contour plane 68 shows the resultant valuesfor I_(A+B)=0.5.

[0046]FIG. 6 shows the qualitative shape of the three dimensionalfunction linking the two segments' input continuous tone values V_(A)and V_(B) to the observed output intensity I_(A+B). For the firstapproach, an input continuous tone value V and an interpolation factor ftogether yield V_(A)=(1−f)V and V_(B)=fV. The closer the interpolationfactor is to 0.5 the greater the difference between the input continuoustone value and the observed output intensity. For V=1.0, this isillustrated in FIG. 6 by the curve 200 on the vertical V_(A)+V_(B)=1.0plane. By definition this curve lies on the function surface. FIG. 6indicates that when any kind of mixing occurs, that is 0.0<f<1.0, theoutput intensity is attenuated, and to achieve the desired outputintensity the sum of the two segments' input values must exceed thedesired output value, that is V_(A)+V_(B)>V. This forms the basis forthe algorithm in appendix A.

[0047] The function shows a linear response when only one segmentcontributes to the output, that is f=0.0 or f=1.0. This assumes ofcourse that the dither matrix includes the effects of dot gain.

[0048] The foregoing description has been limited to specificembodiments of this invention. It will be apparent, however, thatvariations and modifications may be made to the invention, with theattainment of some or all of the advantages of the invention. Forexample, it will be appreciated that the invention may be embodied ineither hardware or software in a suitably programmed digital dataprocessing system, both of which are readily accomplished by those ofordinary skill in the respective arts. Therefore, it is the object ofthe appended claims to cover all such variations and modifications ascome within the true spirit and scope of the invention.

1. In an ink ejection print head comprising a plurality of overlappingprint head segments, at least one means for measurement of the degree ofoverlap between adjacent print head segments; means for providing a halftoning of a continuous tone image and means for adjusting said halftoning means in a region of overlap between adjacent print head segmentsto reduce artifacts between said adjacent segments, said region ofoverlap extending the length of a plurality of ink ejection nozzles andwherein the spatial relationship between adjacent segments is variablewith time, a method for controlling the firing of said nozzles withinthe overlapped segments comprising the steps of: (a) determining ameasure of the overlap between adjacent print head segments; (b)creating a half toning pattern for the nozzles in the region of overlapof the overlapping segments; and (c) adjusting said half toning patternas a function of said measure in the overlapping regions of said printhead segments to reduce artifacts produced by the overlapping of saidprint head segments.
 2. A method for controlling the firing of nozzlesas claimed in claim 1 wherein said step of determining a measure of theoverlap between adjacent print head segments includes measuring thetemperature of the print head segment.
 3. A method for controlling thefiring of nozzles as claimed in claim 1 wherein said step of determininga measure of the overlap between adjacent print head segments includesmeasuring the relative displacement of said overlapping segments.
 4. Amethod for controlling the firing of nozzles as claimed in claim 2wherein said step of creating a half toning pattern for the nozzles inthe region of overlap of the overlapping segments includes employing adither matrix with an interpolation function.
 5. A method forcontrolling the firing of nozzles as claimed in claim 4 wherein saidstep of adjusting said half toning pattern is such that V_(A)+V_(B) isgreater than V where V_(A) and V_(B) are the respective independentdither matrix values of the two adjacent segments and V is thecontinuous tone value to be represented.
 6. A method for controlling thefiring of nozzles as claimed in claim 5 further including the step ofadjusting the firing of nozzles in adjacent segments such thatcorresponding nozzles of respective segments never both fire.
 7. Amethod for controlling the firing of nozzles as claimed in claim 5wherein said further step includes the step of using the inverse of thedither matrix for alternating segments.
 8. A method for controlling thefiring of nozzles as claimed in claim 5 wherein said further stepincludes the step of assigning the output dot to one or the other nozzlestochastically according to a probability given by a currentinterpolation factor of said interpolation function.
 9. A method asclaimed in claim 3 further including the step of ensuring thatcorresponding nozzles of adjacent overlapping segments never bothproduce an output.
 10. An ink ejection print head system comprising: aplurality of spaced apart spatially overlapping print head segments;said region of overlap extending the length of a plurality of inkejection nozzles; at least one means for measurement of the degree ofoverlap between adjacent print head segments; means for providing a halftoning of a continuous tone image and means for adjusting said halftoning means in a region of overlap between adjacent print head segmentsto reduce artifacts between said adjacent segments.
 11. An ink ejectionprint head system as claimed in claim 10 wherein said at least one meansfor measurement of the degree of overlap between adjacent print headsegments includes a means for measuring the temperature of said adjacentprint head segments.
 12. An ink ejection print head system as claimed inclaim 10 wherein said at least one means of measurement of the degree ofoverlap between adjacent print head segments includes a means formeasuring the relative displacement of said adjacent print headsegments.
 13. An ink ejection print head system as claimed in claim 11wherein said means for providing the half toning of a continuous toneimage includes a dither matrix and said means for adjusting said halftoning means includes a summation means having two inputs, one inputbeing the output of said dither matrix and said other input being anoverlap signal derived from said at least one means for measurement ofthe degree of overlap between adjacent print head segments.
 14. An inkejection print head system as claimed in claim 13 further including acomparator means for comparing the output of said summation means and acontinuous tone data input , the output of said comparator means beingthe half tone data for corresponding nozzles of adjacent print headsegments.
 15. An ink ejection print head system as claimed in claim 11wherein said means for adjusting said half toning means in a region ofoverlap between adjacent print head segments includes means forinverting the dither matrix for alternate segments.
 16. An ink ejectionprint head system as claimed in claim 13 further including a means forsubtracting the output of said summation means from a continuous tonedata input signal to produce a half tone data value for driving thenozzles of adjacent print head segments.
 17. An ink jet ejector printhead system comprising: a plurality of spaced apart spatiallyoverlapping print head segments; at least one means for measurement ofthe degree of overlap between adjacent print head segments; means forproviding a half toning of a continuous tone image and means foradjusting said half toning means in a region of overlap between adjacentprint head segments to reduce artifacts between said adjacent segmentswherein said means for providing a half toning of a continuous toneimage and said means for adjusting said half toning means in a region ofoverlap between adjacent print head segments to reduce artifacts betweensaid adjacent segments includes a programmable digital computerprogrammed with an algorithm, said algorithm generating a functionproviding a dispersed dot stochastic dithering reproduction ofcontinuous tone pixel values such that corresponding nozzles of adjacentsegments are never both producing a dot at the same time and the desiredoutput value is less than the sum of the two input dither matrix valuesof adjacent segments.
 18. An ink ejection print head system as claimedin claim 17 wherein said at least one means for measurement of thedegree of overlap between adjacent print head segments includes a meansfor measuring the temperature of said adjacent print head. segments. 19.An ink ejection print head system as claimed in claim 17 wherein said atleast one means of measurement of the degree of overlap between adjacentprint head segments includes a means for measuring the relativedisplacement of said adjacent print head segments.